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Free keywords:
Na-K ATPase; ouabain; Xenopus; oocyte; maturation
Abstract:
uring progesterone-stimulated maturation of defolliculated full-grownXenopus oocytes, the activities of the transport systems forl-alanine, thymidine, chloride, phosphate, and alkali ions decrease. Differences of the extent and time course of these changes suggest that they are controlled by at least partially independent mechanisms.
A closer investigation of the Na-K ATPase has shown that in unstimulated oocytes, ouabain produces maximal inhibition when 8–12×109 molecules are bound per cell. This number is bound during the first phase of a diphasic uptake process. Since this phase can be suppressed by increasing the concentration of external K+ to 45 mmol/liter or more, it is concluded that it refers to binding to the Na−K pump in the plasma membrane. Ouabain bound prior to progesterone-induced germinal vesicle breakdown (GVBD) remains bound after the breakdown, although the Na−K pump loses the capacity to bind ouabain after GVBD in oocytes that had not been exposed to ouabain preceding GVBD. In the presence of Mg++ membranes isolated before regulatory inhibition of pumping and ouabain binding show a Na+-dependent incorporation of 32P from γ-[32P]-ATP that can be reversed by the addition of K+. The phosphorylation site migrates on LiDS-polyacrylamide gel electropherograms at about 98,000 daltons and can be identified as a Commassie blue-stainable band.
